/Engine/Source/Editor/SequencerCore/Public/MVVM/Views/SequencerInputHandlerStack.h
A Reply is something that a Slate event returns to the system to notify it about certain aspect of how an event was handled. For example, a widget may handle an OnMouseDown event by asking the system to give mouse capture to a specific Widget. To do this, return FReply::CaptureMouse( NewMouseCapture ).
typedef [FReply](API\Runtime\SlateCore\Input\FReply)(ISequencerInputHandler::* KeyInputHandlerFunction